1. Field of the Invention
The present invention relates to inspection of materials, and more particularly to the inspection of the surface of materials for creep, metal dusting, irregularities, and manufacturing flaws. With still greater particularity the invention pertains to the inspection of the interior of cylindrical surfaces such as reformer tubes used in chemical processing.
2. Description of the Related Art
Reformer tubes are used in many chemical processes. Examples include tubes used to produce ammonia, methanol, hydrogen, nitric and sulfuric acids, and cracking of petroleum. Reformer tubes, also called catalyst tubes, are one of the highest cost components of such plants both in capital and maintenance. A typical installation consists of several hundred vertical tubes. These tubes represent a significant cost for replacement and can be a major source of plant unavailability if unplanned failures occur.
Such tubes are frequently subjected to pressure changes and contact with corrosive substances. Under such situations creep, metal dusting, and surface irregularities frequently develop. If left untreated, creep will develop into cracks that will propagate leading to failure of the tube.
The plant operator is faced with balancing production needs against tube life and risk of tube failure. The Inner Diameter (ID) of these reformer tubes is generally between 76 mm (3.0 inches) and 127 mm (5.0 inches). During plant operation the catalyst filled tubes are externally heated to allow the reforming reaction to occur. One of the major concerns in plant operation is that the reformer tubes operate at an elevated temperature such that they are susceptible to a failure mechanism referred to as “creep”. This condition exists due to the elevated temperatures and stresses imposed by internal pressure, thermal gradients, and mechanical loading cycles. Being able to identify and locate such damage in its early stages is essential for optimizing plant operation.
Conventional Nondestructive Examination (NDE) inspection techniques currently applied to reformer tubes are geared to finding creep damage in the form of internal cracking. However, with the trend towards larger tube diameters and longer intervals between turnarounds, the detection of such defects may not allow for sufficient time for forward planning of tube replacements. Also, such “end of life” techniques do not allow any differentiation between the “good” tubes. Early detection of underutilized tube life can prevent the lost opportunity on both unrealized production through running them too cool and tube life “giveaway” if good tubes are discarded prematurely.
Typically, destructive testing is used on a small number of tubes removed from the reformer to try and determine the absolute life remaining. Whatever the method is used, the results are used on a sample size that is not statistically valid. It is preferable that all the tubes be surveyed with a NDE technique to characterize their relative condition in order to make sense of the absolute condition assessment provided by the destructive testing.
Reformer tubes undergo creep strain, in the form of diametrical growth, on the first day that they are fired. The ability to accurately measure and record this growth means that the tubes' condition can be monitored on day one. Therefore, not only can individual tubes be retired from service at an appropriate time, but also the reformer as a whole can be assessed for performance.
Another problem that can occur in reformer tubes is metal dusting. Metal dusting is a condition where the process stream attacks the interior of the reformer tube with subsequent, significant metal loss. This can be severe enough to be the life limiting condition for the tube. Typically, the metal dusting damage is limited to a 360° circumferential band around the catalyst tube's interior surface where the critical temperature range exists.
External diameter measurements have been used but they are limited as the automated devices only measure across one diameter and are often access-restricted by tube bowing. Manual measurements are too time consuming to provide more than a few readings per tube. No external measurement method can provide diameter growth data at or through the reformer refractory. External measurements are inherently less precise as they are based on a cast surface rather than the internal machined surface and do not take into account the effects of oxide shedding. The most accurate growth measurements are obtained when ‘as new’ baseline data has been taken prior to the tube being fired for the first time. However, if this is not available by using the top portion of the tube that is operating outside the creep temperature as a reference diameter, the growth profile of the tube can be determined at any stage in its life.
Accordingly, there is a need for an automated method and apparatus capable of examining the internal surfaces of reformer tubes. The method should be nondestructive and provide both absolute and relative information on tube profile.